In traditional stator and rotor structures for fractional and sub-fractional horsepower motors, permanent magnets are often integrated into a rotor assembly that typically rotates in the same plane as a ferromagnetic stator structure that provides magnetic return paths for magnet and current-generated flux. Current-generated flux, which is also referred to as Ampere Turn (“AT”)-generated flux, is generated by passing a current through a coil winding that is wrapped about a pole region of a stator member structure. While functional, conventional stator and rotor structures of these and other electric motors have several drawbacks, as are discussed next.
FIG. 1 illustrates a traditional electric motor exemplifying commonly-used stator and rotor structures. Electric motor 100 is a cylindrical motor composed of a stator structure 104, a magnetic hub 106 and a shaft 102. The rotor structure of motor 100 includes one or more permanent magnets 110, all of which are attached via magnetic hub 106 to shaft 102 for rotation within stator structure 104. Stator structure 104 typically includes field poles 118, each having a coil winding 112 (only one is shown) that is wound about each field pole 118. Stator structure 104 includes slots 108 used in part to provide a wire passage for winding coil wire about stator field poles 118 during manufacturing. Slots 108 also provide magnetic separation between adjacent field poles 118. Stator structure 104 includes a peripheral flux-carrying segment 119 as part of magnetic return path 116. In many cases, stator structure 104 is composed of laminations 114, which typically are formed from isotropic (e.g., non-grain oriented), magnetically permeable material. Magnetic return path 116, which is one of a number of magnetic return paths in which permanent magnet-generated flux and AT-generated flux is present, is shown as being somewhat arcuate in nature at peripheral flux-carrying segment 119 but includes relatively sharp turns into the field pole regions 118.
One drawback of traditional electric motors, including electric motor 100, is that magnetic return path 116 requires a relatively long length for completing a magnetic circuit for flux emanating from one rotor magnet pole 110 and traversing via magnetic return path 116 to another rotor magnet pole 110. Furthermore, magnetic return path 116 is not a straight line, which is preferred for carrying magnetic flux. As shown, magnetic return path 116 has two ninety-degree turns in the stator path. Magnetic return path 116 turns once from field pole region 118 to peripheral flux-carrying segment 119, and then again from peripheral flux-carrying segment 119 to another field pole region 118. Both of these turns are suboptimal for carrying flux efficiently. As implemented, magnetic return path 116 requires more material, or “back-iron,” than otherwise is necessary for carrying such flux between field poles. Consequently, magnetic return paths 116 add weight and size to traditional electric motors, thereby increasing the motor form factor as well as cost of materials to manufacture such motors.
Another drawback of conventional electric motors is that laminations 114 do not effectively optimize the flux density in flux-carrying poles, such as through field poles 118 and stator regions at peripheral flux-carrying segment 119, due to hysteresis losses (or “iron losses”). Hysteresis is the tendency of a magnetic material to retain its magnetization. “Hysteresis loss” is the energy required to magnetize and demagnetize the magnetic material constituting the stator regions, wherein hysteresis losses increase as the amount of magnetic material increases. As magnetic return path 116 has one or more turns of ninety-degrees or greater, the use of anisotropic materials, such as grain-oriented materials, cannot effectively reduce hysteresis losses because the magnetic return path 116 in peripheral flux-carrying segment 119 would cut across the directional orientation of laminations 114. For example, if direction 120 represents the orientation of grains for laminations 114, then at least two portions of magnetic return path 116 traverse across direction 120 of the grain, thereby retarding the flux density capacity of those portions of stator peripheral flux-carrying segment 119. Consequently, anisotropic materials generally have not been implemented in structures similar to stator structure 104 since the flux paths are usually curvilinear rather than straight, which limits the benefits provided by using such materials.
Yet another drawback of conventional electric motors is the relatively long lengths of magnetic return path 116. Changing magnetic fields, such as those developed at motor commutation frequencies, cause eddy currents to develop in laminations 114 in an orientation opposing the magnetic field inducing it. Eddy currents result in power losses that are roughly proportional to a power function of the rate at which the magnetic flux changes and roughly proportional to the volume of affected lamination material.
Other drawbacks of commonly-used electric motors include the implementation of specialized techniques for reducing “cogging,” or detent torque, that are not well-suited for application with various types of electric motor designs. Cogging is a non-uniform angular torque resulting in “jerking” motions rather than a smooth rotational motion. This effect usually is most apparent at low speeds and applies additive and subtractive torque to the load when field poles 118 are at different angular positions relative to magnet poles. Further, the inherent rotational accelerations and decelerations cause audible vibrations.
In another type of electric motor, magnetic poles are positioned at relatively large diameters about (or radial distances from) a rotor shaft. These magnetic poles, as well as the permanent magnets giving rise to those magnetic poles, are typically arranged coaxially about the shaft, with adjacent magnetic poles alternating in polarity. An armature disk usually supports the permanent magnets as separate, non-monolithic magnets in a plane perpendicular to the rotor shaft. Structures such as this are designed based on a certain tenet of electric motor design. According to this tenet, an increase in output torque is achieved by increasing the radial distance between the magnetic poles and the rotor shaft. Consequently, the magnetic poles of this type of electric motor are increasingly being positioned at larger distances from the rotor shaft to increase the torque arm distance from the axis of rotation to the air gaps, thereby increasing the output torque. A drawback to this approach is that additional materials are consumed in forming larger motor structures to accommodate the larger torque arm distance, such as those structures that are used to form magnetic flux return paths. These magnetic flux return paths are typically formed using “back-iron” to complete a larger flux path, which is generally circuitous in nature. By adding back-iron to complete a magnetic circuit, the magnetic material volume through which the magnetic flux passes increases, which detrimentally tends to increase the hysteresis and eddy current losses, both of which can be collectively referred to as “core losses.” Further, the addition of back-iron to complete a magnetic circuit increases the magnetic flux path, thereby exacerbating core losses. Another drawback to motors of this type is that the motor volume increases as the magnetic poles are positioned farther from the shaft, which in turn, limits the available applications and uses for this type of motor.
“Back-iron” is a term commonly used to describe a physical structure (as well as the materials giving rise to that physical structure) that is often used to complete an otherwise open magnetic circuit. Back-iron structures are generally used only to transfer magnetic flux from one magnetic circuit element to another, such as either from one magnetically permeable field pole to another, or from a magnet pole of a permanent magnet to a magnet pole of another permanent magnet, or both. Further, “back-iron” structures are not generally formed to accept an associated ampere-turn generating element, such as one or more coils.
In view of the foregoing, it would be desirable to provide a rotor-stator structure that minimizes the above-mentioned drawbacks in electric motors and generators, and to increase output torque and efficiency either on a per unit size or per unit weight basis, or both, as well as to conserve resources during manufacturing and/or operation.